Every year my hibiscus plants develop circular black spots on the leaves. They yellow, turn brown and fall off. It doesn't seem to hurt the blossoms though.
Certified GKH Gardening Expert
This sounds like it could be a mild fungal infection. I would suggest using a fungicide on the upper portions of the shrub. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
I would also treat the soil with a handful of DOLOMITIC LIME and a teaspoon of WETTABLE SULFUR once or twice each year. This will help to prevent the issue in the future.
This article will help you to grow Hibiscus: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm